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• OAR sparing through FLASH may be achieved in clinically realistic lung treatments. • 30% FLASH enhancement outweighs loss of fractionation and conformality. • Dose and dose-rate thresholds within a beam are critical for clinical translatability. Besides a dose-rate threshold of 40–100 Gy/s, the FLASH effect may require a dose > 3.5–7 Gy. Even in hypofractioned treatments, with all beams delivered in each fraction (ABEF), most healthy tissue is irradiated to a lower fraction dose. This can be circumvented by single-beam-per-fraction (SBPF) delivery, with a loss of healthy tissue sparing by fractionation. We investigated the trade-off between FLASH and loss of fractionation in SBPF stereotactic proton therapy of lung cancer and determined break-even FLASH-enhancement ratios (FERs). Treatment plans for 12 patients were generated. GTV delineations were available and a 5 mm GTV-PTV margin was applied. Equiangular arrangements of 3, 5, 7, and 9 244 MeV proton transmission beams were used. To facilitate SBPF, the number of fractions was equal to the number of beams. Iso-effective fractionation schedules with a single field uniform dose prescription were used: D 95%,PTV = 100%D pres per beam. All plans were evaluated in terms of dose to lung and conformity of dose to target of FLASH-enhanced biologically equivalent dose (EQD2). Compared to ABEF, SBPF resulted in a median increase of EQD2 mean to healthy lung of 56%, 58%, 55% and 54% in plans with 3, 5, 7 and 9 fractions respectively and of 236%, 78%, 50% and 41% in V 100% EQD2 , quantifying conformity. This can be compensated for by FERs of at least 1.28, 1.32, 1.30 and 1.23 respectively for EQD2 mean and 1.29, 1.18, 1.28 and 1.15 for V 100%,EQD2. A FLASH effect outweighing the loss of fractionation in SBPF may be achieved in stereotactic lung treatments. The trade-off with fractionation depends on the conditions under which the FLASH effect occurs. Better understanding of the underlying biology and the impact of delivery conditions is needed. [ABSTRACT FROM AUTHOR]

Cone‐beam CT‐guided single dose of lung stereotactic body radiotherapy (SBRT) treatment with a flattening filter free (FFF) beam and volumetric modulated arc therapy (VMAT) is a safe and highly effective treatment modality for selective small lung lesions. Four‐dimensional (4D) CT‐based treatment plans were generated using advanced AcurosXB algorithm for heterogeneity corrections. 6X‐FFF beam produced highly conformal radiosurgical dose distribution to the target and reduced lung SBRT fraction duration to less than 10 min for a single dose of 30 Gy, significantly improving patient comfort and clinic workflow. Early follow‐up CT imaging results (mean, 8 months) show high local control rates (100%) with no acute lung or rib toxicity. Longer clinical follow‐up in a larger patient cohort managed in this fashion is underway to further validate this treatment approach. [ABSTRACT FROM AUTHOR]